1. Field of the Invention
The present invention relates generally to high speed wireless communication systems, and more particularly, to a method and system for enhancing the quality of a wideband wireless Personal Communication Services by utilizing an adaptive antenna array system.
2. Discussion of the Related Art
The following background is described in connection with a Time Division Multiple Access (TDMA) technology and a wideband Personal Communication Services (PCS) system, as an example. Heretofore, in this field, speedy data transfer forms the cornerstone of modern wireless communication services. Faster communication connections always result in improved system capacity and service quality. Currently, the Global System for Mobile Communication (GSM) based PCS systems operate at 1900 MHz, and support only up to a rate of 9.6 Kbps for data transfer. Higher rate wideband applications are constantly being sought after to meet the ever growing demand of wireless communication services. Accordingly, High Speed Circuit Switched Data (HSCSD) and General Packet Radio Services (GPRS) are being standardized to accommodate this grave need. It is foreseeable that a wideband PCS system, such as one with a 1.6 MHz carrier bandwidth will eventually replace the lower speed system such as the GSM based PCS 1900 system. With a higher speed data service such as a wideband PCS system, however, frequency selective fading is a serious technical hurdle to be overcome. Moreover, since the high rate data transfer requires more information being transmitted through a multi-path radio propagation system, Inter-symbol Interference (ISI) also becomes another major drawback. A current solution for reducing the effect of ISI in a PCS 1900 system is to integrate an equalizer into a receiving system. FIG. 1 illustrates a portion of a typical PCS communication system with the integration of an equalizer. In this part of the PCS communication system, an input signal 10 goes through a receiver filter 12, and is then picked up by a sampler 14 and processed by an equalizer 16. Thereafter, a decision device 18 produces a final output 19. However, this type of communication system using an equalizer will not be feasible in a wideband PCS system due to a unacceptably large path delay difference. In other words, the time delay from the time a first signal will arrive at a receiving device and the time a last signal will arrive through a different path is intolerably big in comparison to the bit duration of a signal. For instance, a typical bit duration in an high rate communication system is on the order of a micro second or tenth of micro second, while a path delay could be on the order of up to 5 micro seconds. Furthermore, Co-channel Interference (CCI) is another technical obstacle for a wideband PCS. In a cellular system, frequencies are reused to increase the capacity, i.e., a frequency band is used in two different cells belonging to different clusters, sufficiently separated so that they don't interfere significantly with each other. In reality, signals using the same radio frequency channel may still unexpectedly infringe upon and weaken each other.
In a TDMA system for wireless services, a single frequency channel is divided into a number of time slots, with each communication signal using one of these slots. With TDMA, an audio signal is digitized, that is, divided into a number of digital packets, each on the order of milliseconds in length. These packets will occupy different time slots. Thus, a TDMA system allows a number of users to share a single radio frequency channel by uniquely allocating the time slots in the channel for each user. For instance, a number of cellular phone users can carry on their conversations simultaneously using the same radio frequency channel to transfer digital packets. This is in contrast to having one user occupy the entire channel for a conversation, to the exclusion of all others. By dividing the radio frequency channel into time slots, a TDMA system increases the capacity of cellular frequencies, but the problem of CCI and ISI become more acute.
Referring now to FIG. 2, a general overview of an adaptive array system 20 for receiving and processing transmitted signals is shown. The adaptive array system 20 typically consists of a plurality of receiving devices 22 such as antennas to detect transmitted signals. A plurality of signal converting devices 24 provide for down converting or demodulating the received signals, and furnishing analog-to-digital conversion. A weight controller 26 is provided for generating a plurality of weights to be integrated into the received signals. Lastly, a combiner 28 is provided for constructing a final output signal 30. Adaptive antenna arrays have been studied in connection with mobile wireless networks to suppress both CCI and ISI. A plurality of weights are generated and integrated into the received signals by minimizing a certain criterion to maximize the signal-to-noise ratio (SNR). Conventionally, a least-mean-square algorithm has been used to update the weights of adaptive arrays, but its slow convergence presents a tracking problem. This could be even more of a problem in a high speed system such as a wideband PCS system. An alternative method such as a direct matrix inversion algorithm has been proposed just for its fast convergence feature. The direct matrix inversion algorithm is not commercially feasible, however, because of its computational complexity. Another algorithm known as constant modulus adaptive algorithm has been considered also. However, the algorithm is problematic in that a PCS system using this algorithm will capture excessive interfering signals.
It would thus be desirable to provide a method and apparatus to combat both CCI and ISI in a wideband TDMA PCS or other similar system.